A magnetic bubble is a magnetic domain characterized by a single domain wall which closes upon itself in the plane of a layer of magnetic material in which it can be moved. Inasmuch as the wall thereabout closes on itself a bubble domain is self-defined and is capable of being moved anywhere in the plane. Magnetic memories employing domains of this type are disclosed in A. H. Bobeck, U.S. Pat. No. 3,460,116, issued Aug. 5, 1969.
Two well-known techniques for moving magnetic bubbles in such memories are commonly referred to as "conductor" and "field-access" drive arrangements. The conductor-access type of bubble memory is disclosed in the above-mentioned Patent of A. H. Bobeck. The field-access type of bubble memory is disclosed in U.S. Pat. No. 3,534,347 of A. H. Bobeck, issued Oct. 13, 1970.
The conductor-access type memory comprises a layer of magnetic material in which magnetic bubbles can be moved. The bubble layer commonly comprises an epitaxially grown garnet material on a nonmagnetic garnet substrate. Several patterns of electrical conductors are formed adjacent the epitaxial layer with the appropriate insulating layers between them. The above-mentioned patent of A. H. Bobeck relating to conductor-access memories discloses an implementation comprising three patterns of undulating electrical conductors in several layers of metallization. The conductors are situated in positions offset from one another along a path of bubble propagation and are pulsed in a three-phase manner to insure directionality of bubble movement as is well understood in the art. The conductor-access arrangement employs intricate electrical conductors which cross one another and are therefore subject to shorts, as well as to defects, which render manufacturing yields of such memory devices low. Moreover, high power requirements characterize all prior art current-access drive arrangements.
R. F. Fischer, U.W. Pat. No. 3,699,551, issued Feb. 16, 1971, discloses a method for the propagation of bubbles which utilizes a "two-phase" conductor-access arrangement employing offset permalloy elements for determining the direction of bubble movement. The permalloy elements are disposed to provide low energy or rest positions for bubbles in positions offset from those to which the bubbles are moved by a pulse applied to one of the conductors.
U.S. Pat. Nos. 3,693,177 and 3,678,479, issued Sept. 19, 1972 and July 18, 1972, respectively, disclose a "single level" conductor-access arrangement for bubbles. The arrangements comprise, in effect, a single undulating conductor which crisscrosses a succession of positions in a bubble path. A bipolar pulse applied to the conductor provides two phases of operation with the single conductor level. The bubble layer itself is of a geometry to provide for bubble offset to rest positions.
The field-access type memory, in contradistinction, utilizes a pattern of permalloy elements which defines a plurality of bubble propagation paths. These elements are arranged in a plane closely spaced apart from the epitaxial layer and are responsive to a uniform magnetic field reorienting in the plane of bubble movement to provide moving magnetic pole patterns which the bubbles follow as is well understood. Half-disc, T-bar, and chevron-shaped elements are all familiar geometries characteristic of field-access type memories.
Field-access arrangements require relatively costly field coils to generate the reorienting in-plane field and also require that that field be provided over a relatively large area necessitating exacting tolerances for the field coils. Moreover, high speeds of operation are difficult to achieve in field-access arrangements because the requisite increasingly larger fields are increasingly difficult to switch.
The conductor-access arrangement on the other hand is difficult to realize because large and uniform electrical conductor patterns are hard to achieve in practice and are characterized by high power dissipation and nonuniformities in current flow. Moreover, even single level conductor geometries when extended to multiple channel operation cause bubble strip out across channels necessitating the separation of the underlying bubble layer into strips as disclosed in the above-noted U.S. Pat. Nos. 3,693,177 and 3,678,479.
The problem thus is to achieve a conductor-access bubble memory with relatively uniform overall current flow, low power dissipation and a single level of metallization.